A federal investigation into Gov. John G. Rowland's activities gained a crucial witness yesterday when a Connecticut antiques dealer with direct ties to the governor agreed to plead guilty to supplying false tax information about his purchase of the governor's Washington pied-à-terre in 1997.

Wayne E. Pratt, an antiques dealer who appears frequently on the PBS television program ''Antiques Road Show,'' has been cooperating with prosecutors since January, lawyers familiar with the investigation say. Prosecutors are trying to determine whether he paid the governor more for the condominium than it was worth.

The plea agreement Mr. Pratt reached yesterday gives him further incentive, lawyers say, to testify on matters important to prosecutors. Those will almost certainly involve Robert V. Matthews, a friend of the governor who was Mr. Pratt's frequent business partner and closest friend.

Mr. Matthews's companies have received $8.7 million in rent and $4.8 million in loans and loan guarantees during Mr. Rowland's tenure. This is the second big break in the government's case since Lawrence E. Alibozek, Mr. Rowland's former deputy chief of staff, pleaded guilty last March to accepting cash and gold for steering state contracts.

Until now, prosecutors had been focusing heavily on the governor's history of letting others pay for renovations to his vacation cottage. But their inquiries have now spread to other facets of his life, including his acceptance of excessive rent and an above-market purchase price for the Capitol Hill condominium.

Lawyers with knowledge of the case said that Mr. Pratt was expected to appear in Federal District Court in Hartford on Thursday to plead guilty to a misdemeanor tax charge.

The potential penalty is one year in prison, but his lawyer, Thomas E. Dwyer Jr., said it was his understanding that his client ''will not be sentenced for two or three years from now, and I expect the sentence to be probation.''

He declined to discuss the condominium purchase, but said that the record would eventually show that ''Bob Matthews took advantage of his friend Wayne Pratt.''

Defense lawyers with clients involved in the investigation called the development highly significant.

''It's the standard way of approaching a case,'' said James W. Bergenn, a lawyer in Hartford. ''You find someone willing to plead guilty and cooperate.''

''There's only been Alibozek before this,'' he said, ''and this helps the federal investigators to the extent that this fellow is credible.''

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He added, ''There is nothing truthful Wayne Pratt can say to contradict that.''

Mr. Pratt, a high-end antiques dealer with showrooms in Woodbury, Conn.. and Nantucket, Mass., bought the tiny condominium during Mr. Rowland's first term in office for $68,500, considerably more than the $30,000 to $50,000 that other units in the same building were fetching at the time.

Mr. Matthews acknowledged during interviews in January that he had helped Mr. Rowland in 1996 realize some value from the Washington condominium he still owned from his days as a congressman. By his own account, Mr. Matthews first arranged for his niece to rent the place, and later suggested that Mr. Pratt buy it.

In an interview in January, Mr. Pratt said the purchase was an ''investment opportunity that was presented to me.'' But real estate records show he sold the 278-square-foot apartment two years later for $30,000 less than he had paid.

Lawyers familiar with the matter said that Mr. Pratt's tax problems stemmed from his having made as much as $7,000 by reporting the loss on his tax return, even though prosecutors have evidence that Mr. Matthews reimbursed him in full. The reimbursements may have taken the form of overpayments on antiques, lawyers say.

Mr. Matthews's lawyer, Hubert Santos, said he would have no comment.

Mr. Matthews, a businessman who has made millions of dollars on real estate and venture capital, has complained publicly that his friendship with the governor has hurt, not helped, his business interests because state agencies strain to avoid any appearance of conflict when dealing with him.

But at least once, the governor himself made a phone call from a golf course in Waterbury in a failed effort to woo a company to Connecticut on Mr. Matthews's behalf. That company was also offered a chance to apply for $2 million in state economic development funds, but it decided against the move.The condominium sale came at an opportune time for Mr. Rowland, who just the month before had bought, with his wife, Patricia, a lakefront cottage in Litchfield. They began major renovations in July, some of which they paid for and some of which were contributed by state employees and people seeking to do business with the state.

Mr. Pratt and Mr. Matthews have been described in news accounts as best friends and business partners. Mr. Pratt had helped Mr. Matthews furnish a house on Nantucket, and was credited with having once helped save his life when Mr. Matthews almost died from a respiratory virus.

Their relationship is currently strained, according to people who know them, because of a partnership they formed in 2000 to buy one of only 14 original copies of the Bill of Rights.

The document, for which they had paid $200,000, had been stolen from North Carolina during the Civil War. Federal authorities stepped in to seize it from Mr. Pratt last year as he was about to sell it for $5 million. To avoid trouble, Mr. Pratt quickly relinquished his claim to the document, now thought to be worth up to $30 million.

Mr. Matthews, in contrast, has no interest in forfeiting his claim to the document. He is suing the federal government and Mr. Pratt for title.

Correction: March 19, 2004, Friday Because of an editing error, an article yesterday about an antiques dealer with ties to Gov. John G. Rowland of Connecticut who agreed to plead guilty to supplying false tax information about his purchase of the governor's condominium misstated the day of the plea deal in some copies. It was Wednesday, not yesterday.

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A version of this article appears in print on March 18, 2004, on Page B00001 of the National edition with the headline: Buyer of Rowland's Condo to Plead Guilty to Giving False Tax Information. Order Reprints|Today's Paper|Subscribe